You can analyze and optimize query performance using the Data Performance view, which provides an aggregated list of entities with long-running queries. A long running query is defined as a query that takes three seconds or longer to complete. Typical examples of a component that can have a long running query is a plug-in with custom FetchXML or a sub-grid or view. When you optimize an entity that has long-running queries, it could significantly reduce the amount of time the query takes to load. Behind the scenes, optimizations add one or more SQL Server indexes.

Applies to Dynamics 365 (online), version 9.x
Applies to Dynamics 365 (online), version 8.1.x

Read the full article at Microsoft Docs: https://docs.microsoft.com/en-us/dynamics365/customer-engagement/admin/analyze-improve-data-query-performance